1. Field of the Invention
This invention relates generally to an acid-resistant calcium carbonate for use in groundwood papermaking systems and related industries, and more particularly is directed to the preparation of a high solids slurry of about 35 wt. % of an acid-resistant precipitated calcium carbonate composition suitable for shipping at an economic rate.
2. Description Of The Related Art
Calcium carbonate, particularly precipitated calcium carbonate, has been used as a filler material in the making of wood-free alkaline paper. Such usage results in a paper with enhanced optical properties, particularly high brightness and opacity, without the expense incurred using titanium oxide fillers, resulting in a much less expensive product. Calcium carbonate, however, cannot generally be used as a filler in acidic paper made from mechanical pulp furnishes because it decomposes in an acidic environment. Since mechanical pulp sheets and newsprint are made in the acidic range, there has long been a need to develop a calcium carbonate composition which is acid stabilized and resistant to decomposition at low pH, so that it can be utilized as a filler material in the manufacture of such paper, such as groundwood paper, where the use of alkaline filler would have a negative impact on the final paper properties.
Paper made from mechanical pulps have been additionally produced under acidic papermaking conditions because of "fiber alkaline darkening" that occurs as pH rises. This means that there is a reduction in brightness of the paper (brightness reversion) when the pH is raised from acid to alkaline in wood-containing systems. Alkaline darkening will occur to some degree in any wood pulps with significant lignin content. The degree of darkening depends on the particular pulps, the pH and the water quality. In general, ground and precipitated calcium carbonate fillers buffer wet end in the 7.5-8.2 pH range. Acid-resistant calcium carbonate compositions thus provide a means for reducing the degree of fiber alkaline darkening and brightness reversion due to their ability to maintain a lower, stabilized pH.
As is known in the art, the acid-resistant calcium carbonate compositions are utilized to improve the optical properties of neutral to weakly acidic paper by its addition to the paper during standard manufacturing processes. Typically, the acid-resistant calcium carbonate composition is added to a first paper furnish containing components necessary for making acidic paper to thereby form a second paper furnish. The amount of calcium carbonate composition which is added to the paper furnish is dependent upon the particular characteristics of the particular furnish, and the desired degree of brightness in the finished product. Representative amounts of the composition for typical paper furnishes range from about 1% to about 30% by weight of the finished product weight. Most preferably, the calcium carbonate compositions are utilized in the range of about 5% to about 20% by weight of the finished product.
Several U.S. patents disclose acid-resistant calcium carbonates. Among these are U.S. Pat. Nos. 5,531,821; 5,584,923; 5,593,488; 5,593,489; and 5,599,388, which patents are assigned to the assignee of the present invention. Other examples of acid-resistant or stabilized carbonate compositions are disclosed in pending patent applications bearing U.S. Ser. Nos. 08/546,493; 08/614,863; and 08/546,145, also owned by the assignee of the present application.
Patent application bearing U.S. Ser. No. 08/614,863 filed Mar. 13, 1996, has recently been allowed. This disclosure teaches the preparation of an acid-resistant calcium carbonate by using sodium aluminate and one or more weak acids to treat the calcium carbonate. This product is produced by ECC International Inc. under the tradename OPTICAL.RTM. NB (neutrally buffered) calcium carbonate.
In U.S. patent application Ser. No. 08/614,863, the acid-resistant calcium carbonate composition comprises a mixture of calcium carbonate with at least about 0.1 percent and perhaps about 1 to 6 percent, based on the dry weight of calcium carbonate, of sodium aluminate, together with about 0.1 percent, based on the dry weight of the calcium carbonate, of one or more weak acids. It was found that the inclusion of sodium aluminate together with an amount of weak acid sufficient to neutralize the composition conferred a higher degree of acid resistance for the calcium carbonate composition in the presence of fiber slurry, and a longer term of pH stability than the prior acid-stabilized calcium carbonate compositions. In one embodiment, the weak acid may consist of a mixture of phosphoric acid together with an organic polymeric acid. The process for producing this acid tolerant calcium carbonate involves first adding to the slurry, containing the precipitated calcium carbonate particles, the sodium aluminate which raises the pH to about 11 or 12, followed by adding the weak acids to the resultant mixture. The resultant mixture is blended for a sufficiently long period of time to ensure uniform mixing of the ingredients, and then allowed to settle.
As stated, this product of U.S. patent application Ser. No. 08/614,863 may be equivalent to the acid-resistant calcium carbonate product of the present assignee, ECC International Inc. (Atlanta, Ga.) and which product is commercially available under the tradename OPTICAL.RTM. NB (neutrally buffered) calcium carbonate. This product is generally shipped to the customer in slurry form having low solids of about 18% to about 20% by weight. Another calcium carbonate product manufactured and sold by the assignee of the present invention, which is not an acid-resistant calcium carbonate, is referred to as OPTICAL.RTM. HB (high bulk). This product is a precipitated calcium carbonate comprised of scalenohedral particles with a median particle size of about 2 .mu.m and a Blaine surface area of 22,000 cm.sup.2 /g. A calcium carbonate with these characteristics may be readily available from other sources in addition to the assignee of the present invention.
In order to produce the product referred to as OPTICAL.RTM. NB, a calcium carbonate similar to the OPTICAL.RTM. HB product having the characteristics discussed in the preceding paragraph is generally chemically treated with 1 wt. % sodium aluminate, based on the dry weight of the calcium carbonate, to impart resistance to the acid dissolution of the calcium carbonate. The pH of the treated slurry is adjusted to 6.8 to 7.0 by the addition of 4 wt. % phosphoric acid. In a manner well-known to those skilled in the art, small amounts of citric acid may be added in the high solids storage tank as necessary to ensure pH stability.
It is thought that it would be more economical to ship this slurry product of the acid-resistant calcium carbonate compositions at 35% solids instead of the 18% to 20% solids by weight, which is presently being shipped. However, in doing this, it would still be necessary to maintain the integrity of the acid-resistant calcium carbonate composition in that the pH of the chemically treated calcium carbonate should remain in the neutral range of between 7.0 to 6.5 over long periods of time during shipping.
Precipitated calcium carbonates which are not acid-resistant, are presently being shipped in a slurry form which is greater than 20% solids, by weight. However, it is not known to the inventors that a high solids slurry of an acid-resistant precipitated calcium carbonate is being shipped while still maintaining the required characteristics of the product performance for particular paper applications.